The present invention relates to compositions containing blends of resins. More particularly it relates to blends of aromatic polycarbonate resins and graft derivatives of ethylene-propylene-diene terpolymers which contain an amount of an ethylene-propylene-diene terpolymer effective to positively upgrade the impact properties, particularly the thick section impact properties, and the resistance to environmental stress crazing and cracking of said blends.
The polycarbonates are well known commercially available materials which, due to their many advantageous properties, find use as thermoplastic engineering materials. The polycarbonates exhibit, for example, excellent properties of toughness and heat resistance.
However, the polycarbonates suffer from two disadvantages. The first disadvantage is the low critical thckness values of polycarbonates, i.e., the thickness at which a discontinuity in Izod impact values occurs. These low critical thickness values tend to limit wall thickness of molded polycarbonate articles to a thickness below the critical thickness. Polycarbonates exhibit notched Izod impact values which are dependent upon the thickness of the polycarbonate articles. Thus, for example, while typical notched Izod impact values of 3.2 mm thick polycarbonate test specimens are generally in the range of about 87 kgf-cm/cm, typical notched Izod impact values for 6.4 mm thick test specimens are generally in the range of about 11 kgf-cm/cm. The relatively high notched Izod values for the 3.2 mm thick polycarbonate test specimens are due to the fact that these specimens are thinner than the critical thickness of the polymer and, therefore, upon impact a hinged or ductile break occurs. The low notched Izod impact values of the 6.4 mm thick polycarbonate test specimens are due to the fact that these specimens exceed the critical thickness of the polymer and, therefore, upon impact a clean or brittle type break occurs.
The second disadvantage of polycarbonates is their susceptibility to environmental stress crazing and cracking. Environmental stress crazing and cracking refer to the type of failure which is hastened by the presence of organic solvents such as, for example, gasoline, acetone, heptane and carbon tetrachloride when such solvents are in contact with stressed parts fabricated from aromatic polycarbonate resins. The most significant effect is a loss in impact strength, particularly thick section impact strength, and an increase in brittle type failure.
Polycarbonate resins have been blended with other thermoplastic materials such as high molecular weight polymeric glycol esters of terephthalic and/or isophthalic acid; acrylonitrile-butadiene-styrene terpolymers, i.e., the ABS resins; and the like. Mixtures of polycarbonate resins with ABS graft copolymers are disclosed to have improved impact properties in U.S. Pat. No. 4,172,103. However, it is disclosed in U.S. Pat. Nos. 3,130,177 and 3,825,393 that impact resistance of polycarbonate resins tends to be lowered when it is blended with ABS resins.
The blending art, particularly when dealing with polycarbonates, is thus a generally complex and somewhat unpredictable area where the empirical approach is still generally the rule rather than the exception. Thus, in order to provide a useful binary blend of a polycarbonate resin with other resins the two resins must be compatible, they must be combinable over certain useful concentrations, and the blend should exhibit a combination of the various advantageous properties of the resins rather than the individual properties of the neat resins. The formulation of blends containing three or more different resins is fraught with a much higher degree of complexity and unpredicatbility.
Grafted derivatives of ethylene-propylenediene terpolymers and compositions of such polymers with other resins are known in the art and are described in the patent literature. It has been disclosed, for example, that olefinic copolymers and terpolymers can be grafted with styrene, acrylo-nitrile-styrene, methyl methacrylate, styrene-methyl methacrylate, and the like, to provide thermoplastics which can be further blended, e.g., with styrene-acrylonitrile, and molded, extruded or vacuum formed into articles having good tensile and impact strengths.
Polymers of this type and methods for their preparation are described in U.S. Pat. Nos. 4,202,948 and 4,166,081, the former being incorporated by reference. Thermoplastic resin blends of polysulfone resins and graft derivatives of ethylene-propylene-diene terpolymers are disclosed in U.S. Pat. No. 3,641,207 to have good processability and impact resistance over relatively narrow concentration ranges.
Aromatic polycarbonate resins and grafted derivatives of ethylene-propylene-diene terpolymers are combinable with each other over a wide range of concentrations and provide compositions which exhibit advantageous properties after molding. Such properties are obtained over a wide range of compositions. Especially noteworthy properties are high gloss on the surface of articles molded from these compositions and improved resistance to environmental stress crazing and cracking as compared with aromatic polycarbonate resins.
While these blends of polycarbonate resin and graft derivatives of ethylene-propylene-diene terpolymers are useful in a wide variety of applications there are certain applications which require greater impact strength, particularly greater thick section impact strength, and better resistance to environmental stress crazing and cracking, particularly at thick sections, than possessed by these blends. It is, therefore, an object of the instant invention to provide polycarbonate/graft derivatives of ethylene-propylene-diene terpolymer blends which possess improved impact properties, particularly at thick sections, and improved resistance to environmental stress crazing and cracking, particularly at thick sections.